Winter Weather Advisory 16, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
601 PM AKST Thu Feb 25 2021

.ANALYSIS AND UPPER LEVELS...

Broad troughing aloft persists over much of the mainland with a
low-amplitude trough moving into northwest Canada. The jet stream
is beginning to amplify as a disturbance propagates through the
flow across the northwest Pacific. There is a decent amount of
moisture associated with the jet as well, currently moving into
the central Bering Sea and toward mainland Alaska.

Around the Cook Inlet region. Weak warm advection off the surface
has promoted the development and building of a stratus deck from
the Susitna Valley southward through upper Cook Inlet. One of the
short term forecast problems is to determine how much this deck
erodes with late-February Sun, and to what extent it returns
overnight tonight. Low clouds and pockets of fog also exist over
the southwest mainland, which should continue to be the case with
ongoing onshore flow.

&&

.MODEL DISCUSSION...
Guidance has been struggling with the first leading jet
disturbance coming into the Bering Sea. There is decent agreement
that there will be at least a weak low/front moving onto the
mainland, but strength is not well agreed upon. The strength will
make a big forecast difference as it will promote a short time
period of southeasterly winds from Anchorage through the Knik
River Valley into the Matanuska Valley. These winds will also have
a strong effect on the high temperatures for the day on Saturday.
Models do come into greater agreement with shifting this energy
into the Gulf of Alaska/Prince William Sound on Monday. The next
model problem is a very strong low and front coming into the
Bering Sea on Saturday/Sunday. There is great agreement of the
existence of a strong low but less agreement on frontal position
and low track given the existence of multiple low centers and the
triple point/occlusion.

&&

.AVIATION...
PANC...The main short term issue with be the stratus hanging
around the Susitna Valley and upper Cook Inlet. The stratus has
almost entirely dissipated, with the remnant deck over the western
Inlet on up to the Susitna Valley. Rapidly advancing high cloud
cover should help to keep the stratus away from the terminal
tonight. The high cloud cover is the leading edge of the next
storm system that may cause some light and intermittent snow
starting late Friday morning, which will persist into Friday
night.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Saturday night)...

An upper level ridge persists over the Gulf tonight, allowing
westerly winds to prevail before transitioning to southwest
tomorrow afternoon behind the ridge. There is still a fair amount
of model inconsistency for a front moving across Southwest Alaska
towards Shelikof Strait. However, the highly amplified upper level
pattern suggests, as mentioned yesterday, that a stronger system
is more likely. As the first front off a low in Southwest Alaska
pushes to the North Gulf Coast, heavy to moderate precipitation,
mainly snow with brief transitions to a rain/snow mix, is expected
along the coast and PWS Friday night through Saturday night.
Models hint at a low developing in northern Shelikof Strait and
moving across the northern Gulf and into PWS by Saturday night.
Gusty winds through Turnagain Arm and along the Hillside are
likely Friday night. There is still some uncertainty about whether
these winds will bend into Anchorage as it will be dependent upon
the track of the low. With models suggesting a stronger storm
track, widespread snowfall across Southcentral is expected by
tomorrow afternoon as multiple waves pass through. Periods of
moderate to heavy snowfall are likely along the eastern Susitna
Valley as well as northern and eastern Copper Basin. Accumulations
of 6-10 inches is possible for these areas Friday night through
Saturday night, with 2-4 inches likely elsewhere and higher amount
at the higher elevations. This system will continue to be closely
monitored for any changes to track and/or strength that may
affect these amount.

There is potential for some lingering snowfall behind the low
across the eastern Susitna Valley and down through the eastern
Kenai through Saturday night. Temperatures across Southcentral are
also expected to begin dropping by Saturday night as colder
northerly winds flow in behind the low.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Thursday
Afternoon through Sunday)...

Broad ridging traversing the area this evening that helped to
produce some fog earlier today has since dissipated. A warm front
will move off the eastern Bering Sea and into the area tonight.
The front will bring warmer air onshore with temperatures near or
above freezing across the Bristol Bay region and bring another
shot of rain and snow to the area. In the Kuskokwim Delta it will
remain in the 20s tonight so all precipitation will be in the form
of snow with an inch or so of accumulations through Friday
morning. Some patchy fog is expected to develop behind this warm
front tomorrow morning and lift by mid-day to early afternoon.

A shortwave moving out of the North Pacific will produce a
surface low and cold front which will bring increasing winds and
more rain and snow for the area. Most of the snow will be in the
Kuskokwim Delta with 2 to 4 inches expected Friday afternoon
through Saturday morning. In the Bristol Bay region a rain/snow
mix will prevail for most populated areas with only minor snow
accumulations through Saturday morning. Blowing snow is possible
mainly along coastal regions of the Kuskokwim Delta from Hooper
Bay to Kipnuk, but uncertainty among model solutions on the
strength and location of surface low makes for low forecast
confidence that blowing snow will occur.

On Saturday, an arctic air mass will spill down from the north
increasing winds and lowering temperatures in the single digits to
below zero with dry weather expected. This cold air will continue
into Sunday with winds decreasing as a ridge of high pressure
moves in from the west.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Thursday
Afternoon through Sunday)...

The upper level jet is crossing the central Aleutians into the
eastern Bering Sea, which is keeping an active pattern intact for
the area.

A warm front draped across the eastern Bering and extending to the
AKPEN will bring a few areas of gales tonight across the eastern
Aleutians and AKPEN with rain and snow. While a low near Shemya
continues to drift north producing some gales across the Western
Aleutians tonight. A shortwave trough moving out of the North
Pacific on Friday will produce a surface low and cold front across
the eastern Bering Sea, which cause small craft winds to develop.
Across the northern Bering the pressure gradient will increase
with gales expected across the northwest Bering Sea Friday. The
threat for heavy freezing spray continues near and west of St
Matthew Island due to the weather pattern.

As the cold front pushes inland Saturday cold air will spill down
from the north across the eastern half of the Bering into the
AKPEN and eastern Aleutians. This will cause gales with storm
force gusts in the northwesterly flow south of the AKPEN. Snow
showers are also likely with some minor snow accumulations
expected across the eastern Aleutians and AKPEN. This trend will
continue into Sunday before winds decrease by Sunday afternoon and
evening as a ridge moves across the central and eastern Bering
Sea.

Saturday a powerful storm force low and frontal system with
hurricane force wind gusts will move across the Western Aleutians
and into the Bering. The system will move east across the
Aleutians and Bering with a wide swath of gales and storm force
winds along with hurricane force wind gusts. The strongest gusts
will mainly be concentrated near the Western Aleutians and the
northwestern Bering Sea.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Bering: A well developed low and front track across the Bering
through Tue. Model confidence is good. Widespread high end gale
force winds and waves with areas of storm force winds and possible
hurricane force gusts with the wind wrapped low and front as it
moves east, diminishing from the west on Tuesday. Waves to 47
feet over the Western Aleutians will be subsiding Monday. Waves to 40
feet are expected over the Bering on Monday, and will subside on
Tuesday.

Gulf of Alaska: A low moves along the North Gulf Coast, with its
front over the Gulf through Tue. Model confidence is good.
Widespread gale force winds and waves with areas of gusty outflow
winds are expected from Kachemak Bay to the Barren Islands,
Shelikof Strait and the Gulf side of the Alaska Peninsula through
Monday. A second front moves into the Gulf with high end gale
force winds through Tuesday. Waves to 22 feet are possible
through the forecast period.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through
Thursday)...

The long-term forecast starts off on Monday with a ridge over the
Alcan border and a broad longwave extending from the Kamchatka
Peninsula to the AKPEN. This pattern is expected to amplify
drastically over the forecast period with a high amplitude ridge
developing over the Bering and a trough deepening over the North
Pacific. On Monday a strong, vertically stacked low is expected
over the central Bering with its front rapidly pushing east into
Southcentral. At this time models develop a triple point low near
Kodiak Island Monday morning. This broad storm system is expected
to bring widespread snow to most of the AOR, with cold
temperatures aloft even supporting snow over the Aleutian Chain
and Kodiak Island. By Tuesday night most of the mainland should
begin to clear out with cold northerly winds developing over
Southwest Alaska and onshore flow providing a chance for long
duration snow over the AKPen. Meanwhile, a large frontal system is
expected to make its way into the western Bering Wednesday
morning and move slowly to the east through Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale: 175 176 178 185-352.
Heavy Freezing Spray: 180 185 412.

&&

SYNOPSIS AND MODEL DISCUSSION...ML
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...KZ/CJ